Magnetic field-dependent proximity sensors are known, as illustrated in U.S. Pat. Nos. 4,719,362 to Nest et. al., 4,587,486 to Soyck, and 4,140,971 to Blincoe. Such proximity sensors typically include a magnet that functions as the target of the sensor, a core made from a material that will magnetically saturate when exposed to a field having a predetermined flux density, and an inductive element, e.g.. a surrounding the core. As the magnet is moved toward the core/inductive element assembly, a distance is reached where the magnetic field of the magnet finds the core to be the smallest reluctance path. As a result, the flux of the field enters the core and, as the distance is decreased, eventually saturates the core. This causes the inductance of the inductive element to decrease. By measuring changes in inductance of the inductive element, the presence of the magnetic field, and hence the position the magnet, may be detected.
Magnetic field-dependent proximity sensors are used in a wide range of applications for detecting when a first movable member is positioned in predetermined spaced relationship to a second member. For instance, such proximity sensors may be used to detect the position of devices used to actuate the flap panels in the wings of an aircraft, as disclosed in U.S. Pat. No. 4,256,277 to Embree. Although magnetic field-dependent proximity sensors used in aircraft typically function satisfactorily, Their performance can be adversely affected when the aircraft is struck by lightning. More specifically, when lightning strikes an aircraft, peak current in excess of 200 Kamps can travel along the skin of the aircraft. These currents generate high frequency electromagnetic fields which may intercept the core and inductor, or the magnet of the target, of a magnetic field-dependent proximity sensor mounted to the aircraft. In some cases, the strength of such fields is sufficient to cause the core to saturate. As a result of this saturation, the inductance of the inductor may fall into a range indicating the magnet, and hence the mechanical element attached thereto, has been moved to within a predetermined proximity of the core and inductive element assembly. Similar change in the detection range of the proximity sensor can occur if the electromagnetic fields demagnetize the magnet of the target. Such erroneous signal information from the proximity sensor can be particularly troublesome when the sensor is used to detect the presence or absence of a mechanical element affecting the safe operation of the aircraft.
In addition to the sensitivity of known proximity sensors to high frequency electromagnetic fields generated by lightning strikes, known sensors also have a tendency to provide spurious results when the sensor is subjected to electromagnetic interference ("EMI") generated by equipment such as electrical motors, wiring and the like positioned near the proximity sensor. Relatedly, known magnetic field-dependent proximity sensors are not typically designed to detect only that portion of a magnetic field having a predetermined direction component. That is, known magnetic field dependent proximity sensors are not generally designed to detect the X component of a magnetic field having X, Y and Z directional components, while at the same time substantially not detecting the Y and Z components of the magnetic field. As a consequence of EMI and the inability of known proximity sensors to discriminate as to the directional component of a field it detects, the resolution and/or target distance to field strength ratio of such sensors may not be as good as is desired.
The actuation zone of known variable reluctance proximity sensors, i.e, the physical region in which the target material must be positioned to be detected by the sensor, is often undesirably small. As a consequence, the respective placement of the target and sensor on the two mechanical elements, the proximity of which is to be detected, is critical to obtain proper proximity detection information. If the sensor and target are positioned too close to one another due to improper installation, mechanical wear, tolerance buildup or other factors, the first mechanical element could contact the second mechanical element during normal operation before the presence thereof is detected. Alternatively, if the sensor and target are positioned too far apart due to the above-noted factors, the sensor will never indicate the first mechanical element is within a predetermined proximity of the second element. Such critically in the relative placement of the sensor and magnet can add significantly to the cost of installing and maintaining the proximity sensor, and can potentially compromise the safe operation of the machine in which the proximity sensor is installed.
Another problem with known magnetic field-dependent proximity sensors is that accurate proximity information is obtained from such devices only in a relatively narrow temperature range. Because such proximity sensors are frequently used in an environment subjected to significant swings in temperature, e.g., in unheated portions of an aircraft, a strong need exists for a magnetic field-dependent proximity sensor that is highly temperature stable.
The weight to detection range ratio of known magnetic field-dependent proximity sensors is typically less than is desired. For instance, a known variable reluctance proximity sensor that is representative of the state of the art with respect to weight to detection range ratios weighs 0.13 pounds and has a detection range of 0.1 inch, providing a weight-to-range ratio of 0.769. This relatively low weight-to-range ratio is especially problematic when the proximity sensor is designed to be used in spacecraft or other equipment where weight is critical.